Usually, a glass plate is widely used as a substrate for displays such as liquid crystal or organic light-emitting diode. However, a glass plate has drawbacks such that its specific gravity is so large that it is difficult to reduce the weight, it is susceptible to breakage, it can hardly be bent and it is required to be thick. Accordingly, in recent years, a plastic substrate has been studied as a substitute for the glass plate. Specifically, display substrates employing e.g. polycarbonate or polyethylene terephthalate have been used.
However, such conventional plastic materials as a substitute for glass have a large coefficient of linear thermal expansion as compared with a glass plate and thus are likely to have such a problem that in a process for vapor depositing a device layer such as a thin film transistor on the substrate at a high temperature, warpage, cracking of the vapor-deposited film or semiconductor disconnection is likely to occur, and their practical use has been difficult.
That is, for such applications, a plastic material having high transparency, high heat resistance, low water absorption and low coefficient of linear thermal expansion is desired.
In recent years, composite materials employing microfibrillated fibers of cellulose such as bacteria cellulose have been actively studied. Cellulose has extended chain crystals and thus is known to exhibit low coefficient of linear thermal expansion, high modulus of elasticity and high strength. Further, it has been reported that by microfibrillating, it is possible to obtain microfibrillated and highly crystalline cellulose nanofibers having a diameter within a range of from a few nm to 200 nm and to obtain a composite having high transparency and low coefficient of linear thermal expansion by filling spaces of such fibers with a matrix material.
Patent Document 1 discloses a composite of bacteria cellulose with a photocurable resin, but by a study made by the present inventors, bacteria cellulose has an average fiber diameter of about 50 nm, i.e. the fiber diameter is large, whereby a light scattering phenomenon is likely to occur. Thus, it has been found that the haze is about 10 as measured by JIS K7136. Also disclosed is a composite comprising nanofiber cellulose fibers (hereinafter referred to simply as “NFCe”) obtained from a wooden material and having a fiber diameter of less than 50 nm and a photocurable resin, but plant-derived impurities are contained in NFCe, and the composite has had a problem that it undergoes coloration when heated.
Patent Document 2 discloses a composite of chemically treated NFCe with a photocurable resin, but according to a study made of the present inventors, the porosity of NFCe is so low that at the time of combining, the matrix material is not sufficiently impregnated among fibers of nonwoven fabric, whereby there has been a problem such that the transparency of the obtainable composite material tends to be low.
Patent Document 3 discloses a composite of bacteria cellulose or cotton with a thermosetting resin. The parallel light transmittance of a material having a cellulose sheet and a resin sheet laminated and pressed as disclosed here, is 81.3% at the maximum, and although the haze of the same sample is not disclosed and is not known, when the total light transmittance of a sample having the highest light transmittance is assumed to be 88.6%, the haze is calculated to be high at a level of 8.2%.
Patent Document 4 discloses a composite of chemically modified cellulose with a cellulose ester, wherein the cellulose ester is mixed with particles of the chemically modified cellulose to obtain a composite material. However, according to a study made by the present inventors, fiberization of cellulose fibers is considered to be inadequate by high pressure homogenizer treatment as disclosed in Example 14 in Patent Document 1, grinder treatment as disclosed in Example 6 in Patent Document 2 or an ultrasonic wave method at 40 W for about 20 minutes in Example 2 in Patent Document 4. Further, after drying the cellulose fibers after microfibrillation, they are combined with acetic acid cellulose to form a composite. However, cellulose fibers once dried tend to agglomerate, and in the composite, they are dispersed as agglomerates, and it is considered that as the diameter of such agglomerates is large, the haze tends to be high.
Patent Document 5 discloses a composite material employing fine fibers of cellulose such as bacteria cellulose. Cellulose has extended chain crystals and thus is known to exhibit low coefficient of linear thermal expansion, high modulus of elasticity and high strength. Further, it has been reported that by microfibrillation, it is possible to obtain microfibrillated and highly crystalline cellulose nanofibers having a diameter within a range of from a few nm to 200 nm and to obtain a composite having high transparency and low coefficient of linear thermal expansion by filling spaces of such fibers with a matrix material.
However, bacteria cellulose has a structure wherein fibers are intricately-intertwined, since bacteria randomly move around while producing fibers. Therefore, bacteria cellulose containing water simply swells, but no fluidity will be formed. Thus, in the production of a cellulose non-woven fabric having a required size and thickness, the production efficiency tends to be poor.
Further, microfibrillated cellulose obtained by treating pulp or the like using by an attrition mill or grinder mill such as a grinder, contains thick fibers and thus had a problem such that even if spaces among such microfibrillated cellulose fibers are filled with a resin, the thick fibers scatter light, whereby no adequate transparency can be obtained.
Patent Document 6 discloses a technique wherein dried microfibrillated cellulose is immersed in water and then irradiated with ultrasonic waves of at least 10 kHz, e.g. 20 kHz, to re-disperse the cellulose. Here, referring to Patent Document 7, it is disclosed that if microfibrillated cellulose (MFC) obtained by passing through a homogenizer under high pressure is in a state suspended in water, there will be a trouble in storage or transportation, and a rotting phenomenon by microorganisms takes place, and therefore, it is required to be dried, but during the drying, MFC tends to agglomerate, and even if the dried MFC is put into water, it cannot easily be dispersed, and therefore, it is irradiated with ultrasonic waves to return it to the initial dispersed state. That is, in this Patent Document 6, ultrasonic waves, etc. are used for the purpose of re-dispersing agglomerated MFC, and it is not disclosed or suggested that microfibrillated cellulose fibers are further microfibrillated by ultrasonic waves. Further, in Example 1 in the Patent Document 6, a dispersion of cellulose after irradiated with ultrasonic waves forms precipitates by a low centrifugal force at a level of 300 rpm (15 G). Such separation by a low centrifugal force shows that cellulose fibers in the dispersion are not sufficiently and uniformly microfibrillated.
Patent Document 8 discloses a method wherein bacteria cellulose is fractured by treatment with ultrasonic waves or the like, and then the obtained aqueous dispersion is spray-dried to produce porous cellulose particles. The ultrasonic treatment in this Patent Document 8 is one for the fracturing treatment to exert a mechanical external force in order to facilitate spray drying of the aqueous dispersion of bacteria cellulose which is already in the form of fine fibers and is not one to further reduce the cellulose fiber diameter itself. Accordingly, even after the ultrasonic treatment, the dispersion is not a uniform dispersion, and the viscosity of the dispersion is high. Further, if the dispersion is diluted with water in order to lower the viscosity of the dispersion, it will be separated into water and fine gel particles.
Patent Document 9 discloses a method wherein non-wooden cellulose fibers are exposed to a high pressure after removing lignin, etc., followed by reducing the pressure to obtain fine fibers, and it is disclosed that centrifugal separation, ultrasonic waves or a pressure filtration method is employed to remove water from the dispersion of fine fibers, but there is no disclosure to obtain fine fibers by microfibrillation fibers by ultrasonic waves themselves.
Patent Document 1: JP-A-2006-241450
Patent Document 2: JP-A-2007-51266
Patent Document 3: JP-A-2006-316253
Patent Document 4: JP-A-11-513425
Patent Document 5: JP-A-2005-60680
Patent Document 6: JP-A-58-206601
Patent Document 7: JP-A-56-100801
Patent Document 8: JP-A-9-132601
Patent Document 9: JP-A-2002-521577